The present invention relates to photoelectric smoke detectors and, more particularly, to photoelectric smoke detectors which are capable of detecting their own deteriorating operability.
In recent years there has been a considerably increased awareness of detecting residential and commercial building fires. Among the many types of fire detectors presently available today, the predominant fire detector is the smoke detector. Smoke detectors rely either upon ionization chambers which utilize a radioactive source for ionizing air to establish a current path between two electrodes and for detecting a decrease in current in the presence of smoke or the photoelectric detector which utilizes a light source and a light responsive device for detecting particles of smoke. Photoelectric detectors in turn are of two basic types. The obscuration detector aligns the light source and light responsive device directly opposite one another and relies upon smoke to interfere with the light falling onto the light responsive device from the light source to change the output from the light responsive device and provide the alarm signal. The second type of smoke detector is the light scattering type which aligns the light source and the light responsive device at an angle with respect to one another so that light from the light source does not fall directly upon the light responsive device; but in the presence of smoke, light is scattered off of the smoke particles and falls on the light responsive device to provide an alarm indication.
In photoelectric smoke detectors, a reduction in source light output or a dirt deposit on the light source or the light sensor will cause the detector to become less sensitive to smoke. The prior art has provided various arrangements for detecting such problems. One such arrangement relies upon a light source and two photocells, one photocell for detecting the presence of smoke and the other photocell for monitoring the optical system itself. The problem with this arrangement is that it requires the use of two photocells and the monitoring photocell does not detect the case where dirt has built up over the detecting photocell. Another such arrangement utilizes a push-to-test feature. In this arrangement, a manually operable lever either scatters light in a light scattering photoelectric detector or obscures light in an obscuration photoelectric detector for simulating a smoke condition. However, this arrangement is not continuously checking and relies upon more complex mechanically movable parts. Still another such arrangement relies upon two different optical paths, one directly from a light source to a light detector and one scattered by smoke particles, but in which both paths are chopped to differentiate between the light received from the monitoring path and the light received from the detecting path. This arrangement requires the use of a more complex light chopper. Still another arrangement useful in a light scattering type photoelectric smoke detector provides a monitoring path between the light source and the light detector. A push-to-test lever normally blocks the light from following this monitoring path but, during a test operation, the lever is operated to allow light from the source to impinge upon the light responsive device to thus test the operability of the optical system. However, this arrangement again relies upon more complex and less economical mechanically movable parts.